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<title>Unnamed Fields (Using the GNU Compiler Collection (GCC))</title>

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<h3 class="section" id="Unnamed-Structure-and-Union-Fields"><span>6.63 Unnamed Structure and Union Fields<a class="copiable-link" href="#Unnamed-Structure-and-Union-Fields"> &para;</a></span></h3>
<a class="index-entry-id" id="index-struct"></a>
<a class="index-entry-id" id="index-union"></a>

<p>As permitted by ISO C11 and for compatibility with other compilers,
GCC allows you to define
a structure or union that contains, as fields, structures and unions
without names.  For example:
</p>
<div class="example smallexample">
<pre class="example-preformatted">struct {
  int a;
  union {
    int b;
    float c;
  };
  int d;
} foo;
</pre></div>

<p>In this example, you are able to access members of the unnamed
union with code like &lsquo;<samp class="samp">foo.b</samp>&rsquo;.  Note that only unnamed structs and
unions are allowed, you may not have, for example, an unnamed
<code class="code">int</code>.
</p>
<p>You must never create such structures that cause ambiguous field definitions.
For example, in this structure:
</p>
<div class="example smallexample">
<pre class="example-preformatted">struct {
  int a;
  struct {
    int a;
  };
} foo;
</pre></div>

<p>it is ambiguous which <code class="code">a</code> is being referred to with &lsquo;<samp class="samp">foo.a</samp>&rsquo;.
The compiler gives errors for such constructs.
</p>
<a class="index-entry-id" id="index-fms_002dextensions-2"></a>
<p>Unless <samp class="option">-fms-extensions</samp> is used, the unnamed field must be a
structure or union definition without a tag (for example, &lsquo;<samp class="samp">struct
{ int a; };</samp>&rsquo;).  If <samp class="option">-fms-extensions</samp> is used, the field may
also be a definition with a tag such as &lsquo;<samp class="samp">struct foo { int a;
};</samp>&rsquo;, a reference to a previously defined structure or union such as
&lsquo;<samp class="samp">struct foo;</samp>&rsquo;, or a reference to a <code class="code">typedef</code> name for a
previously defined structure or union type.
</p>
<a class="index-entry-id" id="index-fplan9_002dextensions-1"></a>
<p>The option <samp class="option">-fplan9-extensions</samp> enables
<samp class="option">-fms-extensions</samp> as well as two other extensions.  First, a
pointer to a structure is automatically converted to a pointer to an
anonymous field for assignments and function calls.  For example:
</p>
<div class="example smallexample">
<pre class="example-preformatted">struct s1 { int a; };
struct s2 { struct s1; };
extern void f1 (struct s1 *);
void f2 (struct s2 *p) { f1 (p); }
</pre></div>

<p>In the call to <code class="code">f1</code> inside <code class="code">f2</code>, the pointer <code class="code">p</code> is
converted into a pointer to the anonymous field.
</p>
<p>Second, when the type of an anonymous field is a <code class="code">typedef</code> for a
<code class="code">struct</code> or <code class="code">union</code>, code may refer to the field using the
name of the <code class="code">typedef</code>.
</p>
<div class="example smallexample">
<pre class="example-preformatted">typedef struct { int a; } s1;
struct s2 { s1; };
s1 f1 (struct s2 *p) { return p-&gt;s1; }
</pre></div>

<p>These usages are only permitted when they are not ambiguous.
</p>
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